My invention relates to welding procedures and particularly to production of welded joints in Aluminum alloys. It eliminates the generation of cracks and also the porosity; or, reduction of cracks and porosity, the common defects, in solidified weld, to a low level.
It is known that welds in Aluminum alloy may have internal macro and also micro cracks and voids. Some cracks may appear clearly on the surface of the weld. Minute cracks which may not appear to the naked eye, may also be discovered by dye penetrant testing. All such cracks are formed because of known reasons like contraction stresses which come into effect as metal cools down, pressures due to gases during solidification, residual stresses. When the micro-structure of the weld has numerous orientations of the crystal growths, because of several directions of cooling gradients, the interdendritic material having a range of freezing temperatures brings about shrinkage stresses, as well as macro and micro voids. It is known that last mass of atoms which solidifies may contain different different alloying elements and impurity elements. This phenomena along with paucity of available molten metal may result in macro and micro voids; and even zigzag lines of demarcation may be observed under microscope. These zigzag lines of demarcation are result of advancing fronts of the freezing molten metal in the weld joint.
Gas entrapped during solidification in the semi-solid state of the weld metal, forms familiar porosity experienced in Aluminum alloy weld micro-structure. Industry has therefore framed standards of acceptance and rejection of gas porosity in Aluminum alloy welds.
The demarcations which may be seen can be, in fact, weak joint in the solidified weld metal. Along with voids and weak joints in the solidified weld metal, stresses which come into existence, form internal micro or macro cracks. Some such cracks may appear on the surface of the weld. And porosity which comes into existence may reduce the strength of the weld joint.
Most specifications of welds do not permit any surface cracks in any classification of welds. Moreover, welds in Aluminum alloy in the manufacture of aircraft and space ship construction and such other applications ought to be without any internal defects, also. There is therefore a need of a controllable process to eliminate or reduce the porosity and cracks to acceptable level in Aluminum alloy welds.
Even though the process is described for Aluminum welds for thin sections, it may be used for welds in thicker sections and also for welds of other similar metals and alloys wherein incidence of higher porosity and/or cracks occur.
My invention is directed towards those aims.